This invention relates to an improvement in a focusing servo control for an optical type data reproducing apparatus such as a video disk player and an audio disk player.
A method utilizing astigmatism as shown in FIG. 1(a) is generally employed for correcting out-of-focusing which occurs due to distortion of the disk or a surface vibration of the disk. According to this method, light A reflected from a disk 1 is caused to produce astigmatism through a cylindrical lens 2, sums of incident light beams in respective diagonal quarter regions in a 4-split photodetector 3 are detected by utilizing the phenomenon that the shape of the light received by the photodetector 3 changes as a, i.e., circle (just focus), b and c, i.e., ellipses in different directions (vertically out of focus) in accordance with a vertical position of the disk 1 relative to the focal plane, difference between these sums of the incident light beams is calculated to obtain voltage corresponding to distance between an objective lens 5 and the disk 1 as shown in FIG. 1(b), and a drive coil 4 is driven by this voltage to move the objective lens 5 and thereby to correct the out-of-focusing.
However, characteristics of a focusing error signal of a commercially available 4-split photodetector are such that, as shown in FIG. 2, the error signal is not symmetrical with respect to the zero point (i.e., just focus) but it is inverted to a negative side in a region which is further from a region e toward a direction going away from the just focus. For this reason, in the focus servo shown in FIG. 1(a), the focusing error signal enters a divergent state i.e., a state in which the focus is far away from the just focus, in a region further away from the region e.
It is, therefore, an object of the present invention to provide an automatic focusing device in an optical type data reproducing apparatus capable of accurately restoring the error signal to a region in which it is adjustable by the focus servo if the error signal enters the divergent state in the characteristics as shown in FIG. 2. According to the invention, the divergent state can be detected by examining whether the error signal is maintained or not in a negative value above a predetermined period of time. If the error signal is within a region in which it can be pulled back to a right position by the servo control (hereinafter referred to as "servo pulling back region"), an integration signal of the error signal is small because the error signal will be momentarily pulled back to the just focus by the servo control. If the error signal has entered the divergent state, the error signal remains a negative value in a relative long period of time because it is out of the servo control range with a result that the integration signal of the error signal becomes a large value. Accordingly, the divergent state can be detected by examining the level of the integration signal of the error signal. Upon detection of the divergent state, an accurate pulling back of the error signal can be again realized by forcibly moving the objective lens to the servo pulling back region.